Superconducting coil fabrication

ABSTRACT

A method of fabricating a superconducting coil is provided which includes fabricating individual coil windings by depositing, shaping and texturing superconductive material in situ on a former which has a substantially curved surface.

This invention relates to superconducting coil fabrication, and tosuperconducting coils so fabricated.

Superconducting coils are currently manufactured by winding lowtemperature superconducting (LTS) wires around formers, and subsequentlyimpregnating them with resin to provide an element of stability andprotection. Reinforcing materials are often incorporated. The mainadvantage of a superconducting coil over a conventional copper-woundcoil is that it consumes almost no power, whilst being able to develophigh fields for a relatively small size. To date, the principalsuccessful commercial applications have been superconducting magnets,such as:—

-   1. high field magnets (up to 21 Tesla) for physics research,    especially solid state physics, and beam steering applications in    particle physics;-   2. high field magnets for nuclear magnetic resonance (NMR) for    molecule identification, especially in the life sciences (eg for    genetics research); and-   3. whole body scanners using magnetic resonance imaging (MRI) for    medical diagnostics purposes, using typically 3 or 4 Tesla magnetic    fields.

The necessity for low temperature refrigeration of any devices using LTSwire has severely limited more widespread use of superconductingmagnets—typical operating temperatures are between 2° and 20° K. Forexample, separation of materials using superconducting magnets is onearea where higher temperature operations would be beneficial, and wouldprobably lead to more widespread industrial uses. Other areas arecatheter steering applications in medicine, and “small parts” MRI—forexamining knees, elbows etc, where the use of “whole body” MRI scannersis unnecessary. Also many potential applications of superconductivity indevices other than magnets eg for the electric power and generationindustry, have not been commercialised because of this requirement forlow temperature operation.

During the last fifteen years since the discovery of high temperaturesuperconductivity (HTS), frequent re-examination of both existing andpotential markets for superconducting devices has taken place, and anumber of demonstrator devices, such as motors and transformers, havebeen built using bismuth strontium calcium copper oxide (BSCCO) tape.Typically, fibres of the superconducting compound are embedded in asilver alloy matrix to form a flexible tape, and there are “dip coating”variants. Unfortunately, the performance of this material is not good inmagnetic fields at the higher temperatures achievable using HTS, and themajor use is likely to be restricted to power cables, because coils andwindings inevitably generate field. Meanwhile, in the field ofelectronic applications, yttrium barium copper oxide (YBCO) thin filmson single crystal substrates have been very successfully demonstrated,and commercialisation is already taking place, eg for narrow bandwidthfilters for the base stations of mobile phone networks, and also insuperconducting quantum interference devices (SQUIDs) for detecting andamplifying minute signals in instrumentation, eg in sensors forinstrumentation used by earth scientists (eg for geophysicalexploration) and in medicine (eg for recording brain activity).

However, single crystal substrates are neither long nor flexible, andthe higher performance of the YBCO films has not yet been utilised inpower devices such as transformers, generators and motors, but there hasbeen a recent very important development. There is currently a majoreffort worldwide to produce long lengths of so-called “coated conductor”which consists of a flexible (usually metallic) tape substrate coatedwith a thin film of YBCO which is highly superconducting even at liquidnitrogen temperatures and in reasonable magnetic fields. Such a tape isshown in FIG. 1, which shows a nickel alloy substrate 1, a buffer layer2 deposited on the substrate, and a YBCO film 3 deposited on the bufferlayer. The layer 2 and the film 3 are deposited using techniques such assputter coating, thermal evaporation or chemical vapour deposition(CVD). The substrate 1 is 10-100 microns in thickness, so as to beflexible but mechanically strong, the buffer layer is substantially 1micron in thickness, and the YBCO film 3 is 1-5 microns in thickness forhigh Jc, high Je, excellent performance “in field”, tight minimum radiusbend and good mechanical stability.

As far as we are aware, no coils of coated conductor have beensuccessfully wound, but this will certainly be possible whensufficiently long lengths of high-grade material become available.However, this “upscaling” process is likely to take several years, andmeanwhile BSCCO tapes must be considered for high field applications,and comparatively low temperature operation will still be necessary.Even when YBCO tapes do become available in sufficiently long lengths,there is the drawback that YBCO films are, of course, ceramic, andtherefore brittle in nature. This limits the amount of strain they cantolerate during coil manufacturing processes, including reeling andwinding operations.

Thus, in the case of the well known “pancake” coil (see FIG. 2) layersof tape, insulated from each other, of course, are wound precisely ontop of one another, and there is no lateral or shear strain on the tape.The winding thus resembles the winding of audio recording tape onto areel of an old-fashioned tape recorder.

Each loop of winding generates a contribution to the overall magneticfield at the centre of the coil, the contribution of each loop beingproportional to the current in the coil, All the loops are in series,and carry the same current, but the outer ones are at a greater distancefrom the coil centre and cannot make as much contribution. They are,therefore, progressively less effective in contributing field. Also, inmost applications a more uniform field is required over a larger volume.A stack of pancakes with the same central axis is one way of achievingthis but a more useful configuration for magnets, and for other deviceswith windings, is a “solenoid” coil (illustrated schematically in FIG.3), where wire or tape windings 4 extend along a cylindrical former 5.In the conventional wire-wound case, the coil resembles a cotton reel,but the aspect ratio can easily be varied.

In FIG. 3, one layer of a helical tape winding is illustrated with nooverlap between the turns. Of course, if the tape is insulated, theturns may overlap, just as the wire turns overlap in a conventionalsolenoid. Each successive layer when winding a solenoid necessitates a“layer turn”, which is easy to do with wire, but much more difficultwith tape, because of the necessity to subject the tape to shear forcesin a controlled way so as not to destroy its integrity. In the case ofYBCO coated tape, this is a major problem because of the brittle natureof the layers, and the degrading of the superconducting performance ifany cracks are present.

There is a further problem in the case of LTS windings, whether solenoidor pancake, for very high field magnets. As successive windings on theoutside of the coil are added, their effect on the field at the centreis further diminished by the fact that the inner windings are now bathedin the higher field, and this reduces the critical current they cancarry, which in turn diminishes their contribution to the magneticfield. Thus, adding further outside turns to the coil, which requiressubstantial lengths of high quality wire, becomes progressively lesseffective at increasing the desired field. This “law of diminishingreturns” has severe economic consequences for high field magnets forphysics research and high field NMR magnets, and is the major drivingforce for examining the possibility of HTS materials, where the criticalcurrents can be much higher in field.

An aim of the invention is to provide alternative, and improved, routesto the fabrication of superconducting coils.

The present invention provides a method of fabricating a superconductingcoil, the method comprising the step of fabricating individual coilwindings by depositing, shaping and texturing superconductive materialin situ on a former which has a substantially curved surface.

Preferably, the former defines a substantially cylindrical surface. In apreferred embodiment the former defines a substantially right circularcylindrical surface.

Advantageously, the method further comprises the step of depositing andshaping buffer layers between successive coil windings.

Preferably, the superconductive material is deposited on the former by afilm deposition technique, and the buffer layers are deposited by a filmdeposition technique.

In a preferred embodiment, the method comprises an initial step offorming a spiral of textured buffer layer on the former, and thetextured buffer layer is formed by helically positioning a flexibletextured tape onto the former.

In another preferred embodiment, the spiral textured buffer layer isformed by a film deposition technique.

In a preferred embodiment, the film deposition technique includes thestep of forming a spiral of textured buffer layer on the former,depositing a superconductive layer over the spiral buffer layer to forma first coil winding, depositing a second buffer layer onto thesuperconductive layer, and depositing a second superconductive layeronto the second buffer layer, thereby forming a second winding of thecoil, and repeating the buffer layer and superconductive layerdepositions to form as many coil windings as required, each depositionprocess being such as to transfer the texture of the underlying layer tothe newly-deposited layer. In this case, the spiral of textured bufferlayers may be written onto the former using the IBAD technique using afixed ion beam or the alternative methods of ISD, IAD or any othersuitable variant and rotating and translating the former. Alternatively,the deposition source (eg the ion beam) is translated and the former isrotated but not translated. Another possibility would be to form abuffer layer completely overlying a textured cylindrical former, andthen removing a spiral track of the buffer layer (for examplelithographically) to form the spiral of textured buffer layers.

Conveniently, each of the superconductive layers is an YBCO layer.Alternatively, each of the superconductive layers is any other suitablesuperconducting film such as a rare earth barium copper oxide (ReBCO)film.

Advantageously, after the addition of a predetermined number of coilwindings, a reinforcement shell is formed on the coil, a textured spiralis formed on the surface of the reinforcement shell to define a furtherbuffer layer using the same process as used on the original bufferlayer, and then further coil windings are deposited by the addition ofsequential superconducting and buffer layers.

Preferably, the method further comprises providing connecting links toconnect the ends of adjacent coil windings. Conveniently, the connectinglinks are provided within the former, and each includes a fault currentlimiter.

Advantageously, the deposition steps take place in a film depositionchamber, and deposition of the buffer layers occurs from one side of thechamber, and deposition of the superconductive layers occurs from theopposite side of the chamber, and the former is rotated during thedeposition process. The two sides of the deposition chamber may beseparated by baffles.

In another preferred embodiment, the former is provided with a texturedcylindrical surface, and with a spirally-wound heater winding wire, theturns of which are spaced by a means of spacing, the means of spacingbeing the diameter of the wire or grooves in the former, a first bufferlayer is deposited to form a spiral buffer layer track between the turnsof the heater winding, an super-conductive layer is deposited over thefirst buffer layer, and a further buffer layer is deposited on top ofthe superconductive layer to form a first coil winding, a second heaterwinding wire is wound between the turns of the first heater windingwire, the first heater winding is removed, and a second coil winding isformed, the second coil winding being constituted by a first depositedbuffer layer, a deposited superconductive layer and a second depositedbuffer layer, and the process is repeated to form additional coilwindings as required, each deposition process being such as to transferthe texture of the underlying layer to the newly-deposited layer. Interms of texture or superconducting performance.

The method may further comprise the step of circulating coolant withinthe former, and/or of testing, in situ, each coil winding.

The invention also provides a method of making a multilayered texturedsuperconducting coil, the method including the steps of fabricatingsuperconductor coil windings and insulating layers by film depositionmeans, whereby the films are patterned by masking or machiningoperations before or after film deposition, in-situ or ex-situ, and/orby subsequently patterning using lithographic techniques allowing atailoring of coil properties by controlling the geometry (width,thickness, spacing, or pitch) of the superconducting paths at everypoint on the surface.

The invention further provides a method of fabricating a superconductingcoil, the method comprising the step of helically positioning a flexiblecoated tape onto a substantially cylindrical former, the coated tapebeing constituted by an insulating buffer layer insert on a flexibletape substrate and a coating of superconductive material.

Advantageously, YBCO constitutes the superconductive material, and thetape is a textured substrate made, for example, by the RABiTS process.

Preferably, the former is generally barrel-shaped, with tapered portionsat each end of a cylindrical control main portion.

The invention will now be described in greater detail, by way ofexample, with reference to FIGS. 1 to 15 of the drawings, in which:

FIG. 1 is a schematic representation of a conventional long lengthcoated conductor consisting of flexible tape substrate coated with athin film of YBCO;

FIG. 2 is a schematic representation of a conventional pancake coilconsisting of layers of tape insulated from each other;

FIG. 3 is a schematic representation of a conventional solenoid coil;

FIG. 4 is a schematic representation of a first form of coil constructedin accordance with the invention, the coil being shown at an early stageof construction;

FIG. 5 is a diagram showing the magnetic field geometry of the coil ofFIG. 4;

FIG. 6 is a schematic representation of a coated conductor tape whichcan be used to form the turns of the coil of FIG. 4;

FIG. 7 is a schematic representation of a second form of coilconstructed in accordance with the invention, the coil being shown in anearly stage of construction;

FIG. 8 is a schematic representation of a cylindrical former for usewith the coil of FIG. 7;

FIG. 9 is a schematic representation of apparatus for forming a modifiedversion of the coil of FIG. 7;

FIG. 10 is a schematic representation illustrating the formation of athird form of coil constructed in accordance with the invention;

FIG. 11 is a schematic representation of apparatus for fabricating acoil of the type shown in FIG. 7;

FIGS. 12 a and 12 b are schematic representations of apparatus fordirectly inducing texture on cylindrical surfaces;

FIG. 13 is a schematic representation illustrating the fabrication of acoil on a cylindrical surface of the type shown in FIG. 12;

FIG. 14 is a schematic representation illustrating the turns of a coilconstructed in accordance with FIG. 13; and

FIG. 15 is a schematic representation illustrating a four-layer coil.

Referring to the drawings, FIG. 4 illustrates an early stage ofconstruction of a first form of superconducting coil. This coil isformed by winding a YBCO coated tape 11, or a tape coated with amaterial other than YBCO which exhibits similar high temperaturesuperconducting properties, or other form of superconducting tape egpowder in tube (PIT) BSCCO onto a generally cylindrical former 12. Theformer 12 has tapered portions 12 a at each end thereof, so that theformer is generally barrel-shaped. The tape 11 is helically wound ontothe former 12 so that, as the tape encounters a tapered end portion 12a, the “outer edge” of the tape (the edge furthest away from thedirection of translation of the winding) will experience a small forcetending to impart a slight twist, which naturally takes the tape back inthe direction from which it came. As shown in FIG. 4, the pitch of thewinding consequently narrows so that, after a few turns on the taperedend portion 12 a, the tape will start to “climb” the taper and thetransverse direction of winding will have been reversed. Consequently, asecond layer of winding automatically starts without the need for anyexcess sheer strain in the plane of the tape, or complex windingcontrol. Winding of the tape 11 continues until the tape encounters theother tapered end portion 12 a which, in a similar manner, reverses thedirection of winding of the tape. The winding of the tape continuesbackwards and forwards until the winding is complete. It should benoted, here, that the direction of the current which will flow in thecompleted winding will be the same for all winding layers. This shapedformer 12 facilitates the winding of the superconducting tape 11,enabling the production of a multi-layer coil using a long continuouslength of tape without undue strain on the tape as it is wound.

By appropriate choice of the angle of taper, and the precise geometry ofthe former 12, commensurate with the geometry of the tape 11 (whosewidth and thickness are important factors), the correct aspect ratio ofthe coil can be preserved. The more closely-spaced windings at the endof the coil will impart an “end correction” to the magnetic field nearthe coil ends. The effect of this is to make the field lines straighter,and hence more uniform over a greater volume, as is illustrated in FIG.5, which shows the magnetic field geometry of a coil constructed in thismanner. Thus, the field lines emanating from a normal uniformally-woundsolenoid resemble those from a long bar-shaped permanent magnet.Increasing the current density at the end of the solenoid, by decreasingthe winding pitch as shown in FIG. 4, straightens out (strengthens) thefield lines, thus increasing the uniformity of magnetic field for thesame length of coil. A tailored “end correction” can thus be achieved.The magnitude of the effect depends on the angle of taper of the endsections 12 a of the former 12, and the width and thickness of the tape11 relative to the winding pitch.

The YBCO coated tape 11 must be very well textured, that is to say itsstructure should be as near to that of a single crystal as possible. The“c” axis of all the grains must be aligned in essentially the samedirection, close to the normal of the plane of the film deposition, andthe number of high-angle grain boundaries in the ab-plane must also beminimised, since these act as a “weak links” or obstructions to thepercolative supercurrents. Such obstructions are to be avoided, as thecurrent tries to go around them.

In order to achieve this well-oriented texture, the flexible substrate 1(see FIG. 1) itself must be textured (using, for example, the so-calledRABiTS approach or a variant of this approach) thus imparting texture tothe subsequent buffer layer 2 and the YBCO layer 3. Alternatively, ifthe substrate 1 is randomly oriented, then some means must be found toproduce texture for a buffer layer sequence on top of which an orientedYBCO layer 3 can be deposited (using, for example, the so-called IBAD orIAD or ISD approaches or variants of these approaches). Otherwise, thesuperconducting properties will resemble those of bulk material, ratherthan the much higher performance thin films deposited on single crystalsfor superconducting electronics applications.

FIG. 6 shows a section of YBCO coated tape made by the RABiTS process.This process consists of fabricating a textured substrate 12 by a seriesof rolling/reduction operations and heat treatments on nickel and nickelalloys, which have the correct cubic latice and atomic spacing forsuccessful subsequent YBCO growth (as indicated by the reference numeral13). Buffer layers between the substrate 12 and the YBCO layer 13 arerequired to prevent diffusion of unwanted chemical species between thesubstrate and the YBCO layer and vice versa. Typical buffer layers arecerium oxide (CeO) and yttrium stabilised zirconia (YSZ), palladium(Pd), silver (Ag) or any other suitable material which exhibits thephysical and physicochemical properties required of the buffer layer.The result is a tape, typically 50-100 microns thick, having excellenttexture, with the grains all having their c-axes pointing in the 100direction, and having very little in-plane mis-orientation in addition.

Another known way of obtaining the necessary texture is to start with ahighly-polished surface (typically an Inconel tape) and to deposit, forexample, YSZ in the presence of an ion beam impinging on the substrateat a specific angle, which has the effect of inducing texture in thegrowing YSZ film. Shadowing effects and channelling effects have beensuggested as explanations for this textured growth on amorphous orpolycrystalline substrates; and, in one technique, sometimes referred toas ISD (inclined substrate deposition), evaporation or pulsed laserdeposition of buffer layers is achieved, again at specific angles, butwithout the need for an ion beam. Schemes using sputtering to producethe required conditions at the surface have been studied also —ionassisted deposition (IAD) is one example. The aim is always the same—toproduce a textured layer on top of which highly superconducting YBCOfilms can be grown. The advantage of starting with an untexturedsubstrate is that the substrate can be mechanically very strong and,therefore, thinner; which means that the engineering current density canbe higher than for a RABiTS coated conductor for the same YBCO filmproperties. (“Engineering current density, Je” equals the total currentcarried by the film, Ic, divided by the whole cross-sectional area ofthe conductor, including that of the substrate. It is, therefore,considerably less than the current density of the YBCO film itself,where Jc equals Ic divided by the cross-sectional area of the filmonly.)

Whilst YBCO films have much higher current density than BSSCO tapes,this current density is “diluted” by one or two orders of magnitudebecause the substrate thickness is typically 10 to 100 microns, whereasthe typical YBCO film thickness is one micron, and Jc is 10E6Acm-2.Nevertheless, the engineering current density is still adequate for manypurposes; and, if the temperature is reduced, the current carried by theoverall conductor structure will increase considerably. BSSCO tapes canbe manufactured in long lengths and are commercially available, but theydo not exhibit “true” superconductor behaviour because they have someresidual resistance and cannot maintain “persistent” currents.Nevertheless, they are suitable to manufacture coils of the type shownin FIG. 4, even though not as suitable as YBCO coated tapes.

FIG. 7 illustrates an early stage of construction of a second form ofsuperconducting coil. This consists of the “writing” of a spiral 21 oftextured buffer layer on a cylindrical former 22 using the IBADtechnique, by using a fixed ion beam (indicated schematically by anarrow 23) and rotating the former at the same time as translatingit—resembling a screw thread cutting operation using a lathe with a“lead screw” which feeds the cutting tool into a work piece. This stepcan be followed by the MOCVD (metal organic vapour deposition) or otherprocess to deposit a YBCO layer (not shown) over the entire cylindricalsurface of the former 22, whilst the former is rotated for uniformity ofdeposition. Where the YBCO layer overlies the spiral buffer layer 21, itcopies the texture of that underlying buffer layer, and so will behighly superconducting. Where the YBCO layer is deposited onnon-textured regions, the superconductivity is two orders of magnitudelower, and can be ignored. In this way, the first layer of a solenoidcoil is formed. If necessary, a laser or mechanical scriber (not shown)can be used to cut an isolating region between the highlysuperconducting turns.

Another insulating buffer layer (not shown) is then deposited, using anyone of a variety of techniques—without using the slow IBAD process,because the buffer layer copies the form of the surface which itoverlies. Thus, where the buffer lies on a textured YBCO surface, thebuffer will have a textured surface; and, where the buffer lies on anuntextured YBCO surface, the buffer surface will be untextured. AnotherYBCO layer completes the second “winding” of the solenoid, as the newYBCO surface copies the form of the buffer layer which it overlies, andso on until a multilayer solenoid, consisting of concentric cylindricalshells, is built up without the need for a long textured tape. It willbe appreciated that the slow IBAD process is only used for the initialtextured buffer layer, and after the provision of each reinforcementshell.

Of course, the coil layers have to be connected together, either inseries or in parallel, and pre-deposited tracks of YBCO or even BSSCOtapes located inside the former 22 may be used to make this connection.FIG. 8 illustrates schematically the former 22 provided with connectinglinks 24 connected to bond pads 25 provided at the ends of theindividual windings. The connecting links 24 may be short lengths oftape located within the former 22 on the inside or the outside, beforeor after film deposition, or they may be tracks of YBCO film depositedbefore the main coil deposition. The former 22 may be grooved, inside oroutside, to accept the links 24, and there may be slits cut at an angleat the ends of the former to allow connection of the tracks to the coilends. A shaft encoder (not shown) is used to control accurately theexact rotational position of former 22, thereby permitting precisecontrol of the start and end of each of the “windings”, and henceco-location of these starts and ends with the bond pads 25.

For some applications, it would be preferable to introduce someprotective circuitry, again integral with the coil, in order to protectthe device, or part of the device, against a superconductivity “quench”.For example, if the coil were to be the inner coil of a much larger,conventional superconducting magnet in order to boost the latter'sfield; then, if the outer sections quenched, the so-called “insert coil”would experience a massive rate of change of flux and hence a massiveinduced current. This would lead to the destruction of the device.

In order to prevent this happening, the arrangement of FIG. 8 ismodified by including an FCL (fault current limiter) in each layer ofthe coil. Such FCL devices exist in thin film form already, and could beintegrated with the coil, either by patterning the ends, or by making“weak links” (layer connections) between the layers, perhaps positionedinside the former 22 before deposition begins. These weak links willfail before the layers fail, thereby preventing the occurrence of themassive induced currents referred to above. This is because an FCLpermits the passage of only a predetermined amount of current, and sowill fail before coil destruction, provided the predetermined current ischosen to be less than that which would lead to coil destruction.

A modification of the coil fabrication technique described above withreference to FIG. 7 would be to use the length of textured tape (coatedor uncoated) to establish the initial textured spiral buffer layer 21.This layer 21 would, therefore, resemble the winding shown in FIG. 3.

A further embodiment of the invention, which is described, by way ofexample, for the coil fabrication technique described above withreference to FIG. 7, would be to introduce a shell to reinforce themechanical strength of the coil, and to ensure that after a number ofmulti-layer coil windings the textured region does not degrade. Thereinforcement shell could be deposited after the deposition of a numberof buffer and superconducting layers, say for example ten, where thecross-section of the coil would appear much like that of FIG. 2,consisting of concentric cylindrical layers. The outside surface of thereinforcement shell, once deposited, would require texturing using thesame technique as for the texturing of the initial buffer layer. Onemethod that could be used to deposit the reinforcement shell would be toposition the shell during a shrink-fitting operation. The coilfabrication technique proceeds as described above, until a similarnumber of layers are deposited, say for example ten. This process canthen be repeated to produce concentric reinforcement shells within aconcentric solenoid, constituting the coil.

The sections of the concentric solenoid between the concentricreinforcement shells require an interconnection scheme, as between theindividual layers of the superconducting coil described above. Thisinterconnection scheme would take the form of an FCL device, which wouldbe more economical than having an FCL for each layer of thesuperconducting coil. As described above for the FCL devices integratedwith each layer of the coil, the FCL devices for the section ofconcentric solenoid would decouple sections of the coil in the event ofa superconducting quench.

FIG. 9 shows schematically apparatus for forming a modified version ofthe coil of FIG. 7. This apparatus comprises a film deposition chamber31 which houses a rotatable cylindrical former 32 which is similar tothe former 22 of FIG. 7. The former 32 is rotatable in the direction ofthe arrow 33, and the chamber 31 is divided into two portions by meansof baffles 34. A textured buffer layer (of for example CeO) is depositedon the former 32 by a “line-of-sight” evaporation process, indicatedschematically by an arrow 35, whilst YBCO deposition proceeds by asimilar “line-of-sight” evaporation process on the other side of therotating cylindrical former, as indicated schematically by an arrow 36.The baffles 34 help maintain the required differential pressures in thetwo portions of the chamber 31.

As the former 32 is rotated, the depositions proceed in parallel withthe result that a “Swiss roll” geometry of YBCO and insulating bufferlayers is formed. This process is ideal for forming a “pancake” coil.

FIG. 10 illustrates schematically a coil formation technique which isintermediate the “long coated conductor” technique of FIG. 4 and thedirect in situ film deposition technique of FIGS. 7 and 9. In this case,a coil is formed in situ by unwinding a reel of a textured flexiblesubstrate 41 formed using the RABiTS approach, and rewinding the sheetonto a former (not shown) to fabricate the coil. The sheet 41 could haveboth sides thereof formed with a textured coating.

Bearing in mind that deposition temperatures play an important role inthe formation of film layers, and that the mechanical properties of along coated tape which is used as an initial winding layer varysignificantly with temperature, this intermediate technique can proveadvantageous compared with the two-stage process described above.

In any fabrication process using film deposition techniques, heattreatments are inevitably involved, either simultaneously with layerdeposition, or after the film deposition process, and the control ofoxygenation conditions during such heat treatments is crucial. Ratherthan having a single film deposition chamber, and having to insert andwithdraw the former many times during the coil fabrication process, itis preferable that different processes are carried out in differentinterconnected chambers. Thus, as shown in FIG. 11, the apparatus ofFIG. 9 could be modified to provide three contiguous treatment chambers51, 52 and 53. The cylindrical former 54, upon which a coil is to befabricated, can be shuttled to and from the various chambers 51, 52 and53 for different treatments, such as film deposition, heat treatment,oxygenation, formation of layer interconnects etc. Thus, the chamber 51could be for buffer layer deposition, the chamber 52 for YBCOdeposition, and the chamber 53 for brazing/handling operations for theinterconnects between layers etc. Each of the chambers 51, 52 and 53 isprovided with a window (not shown) to allow operator intervention, butit will be appreciated that the majority of processes are under computercontrol. One of the chambers could also be used for forming a texturedsurface on the cylindrical former 54, using any of a number ofprocesses, including “melt-spinning”, where a surface is rotated rapidlywhile molten metal is sprayed onto it, thereby ensuring that rapidsolidification results in a textured finish.

As described above, the different chambers, 51, 52 and 53 of theapparatus of FIG. 11, can be used to apply different treatments andprocesses to the coil as it is made. This method is one embodiment ofmany that can be used to process and heat the coil during itsmanufacture. One aim of these processes and treatments is to control theoxygen content of the coils. The oxygen stoichiometry in the layersaffects the properties of the superconducting coils. If oxygen ispresent in the wrong proportion, with respect to the other constituentelements of the superconducting layer, the, superconducting coils willno longer superconduct. So the oxygen content needs to be present withinprecise limits. However, depending on temperature and chemicalpotential, oxygen moves within the atomic lattice structure of thebuffer and superconducting layers of the coil, and the coil structurecould contain buffer layers that are oxygen sources, oxygen sinks,oxygen gateways, oxygen barriers and, as the situation demands, layersthat are permeable to oxygen. The properties of the buffer layers areparticularly dependant upon the temperatures of their deposition, theirheating rates, their cooling rates immediately after deposition, andtheir other time-temperature histories during processing.

The most likely geometry for coil fabrication using the film depositiontechnique is to carry this out on a rotating cylindrical former providedwith an integral, preferably internal, heating element. This arrangementwould ensure that most of the heating requirements are supplied to raisethe temperature sufficiently to ensure the correct epitaxial depositionof layers. If necessary, an additional radiation heating arrangementcould be supplied. The temperature during a film deposition process mustbe closely controlled, and this is particularly difficult where a longflat conductor is being processed. However, as is the case with filmdeposition of a cylindrical former, the presence of an axis of symmetrylends itself to a much better control of uniform heating, for example,by incorporating heating elements inside the cylindrical former, orembedding these in the walls of the cylindrical former. Such heatingelements may be, for example, tungsten coils or halogen lamps.Complementing this heating arrangement by the use of radiation heatingimpinging on the external surface of the cylindrical former is alsouseful for process control, with the main heating being done internally,but the “fine tuning” heating being carried out using external radiationheating methods.

FIGS. 12 a and 12 b are schematic representations of apparatus forforming coils by directly inducing texture on cylindrical surfacesincluding the process of extrusion. FIG. 12 a shows a cylindrical former61 around which a sheet 62 of RABiTS material has been wrapped. Thesheet 62 is then subjected to rolling and heat treatments using a second(heated) cylinder 63, this treatment eliminating the join in the sheet.Alternatively, the two cylinders 61 and 63, thus being a process ofperforming the texturing in situ.

FIG. 12 b shows a modification of the apparatus of FIG. 12 a, in whichthe cylinder 61 rotates inside a drum 63 of large diameter. Here again,a sheet of RABiTS material 62 may be wrapped around the cylinder 61, andthe join eliminated by rolling and heat treating, or the texturingprocess maybe carried out in situ by rotating the two cylinders 61 and63 relative to one another under pressure to induce texture. Thearrangement of FIG. 12 b is actually preferable to that of 12 a, as thestresses formed at the surface of the cylinder 61 are more like theconditions reached during the rolling of RABiTS tape.

In another modification of the apparatus in FIG. 12 a, the material onthe surface of the cylinder 61 is subjected to rolling and heattreatments by a series of cylinders, each of which, as cylinder 62, isin contact with the surface of cylinder 61. The series of cylinderstexture the material on the surface of cylinder 61 by extrusion in thesense of swaging or drawing the surface of cylinder 61 through a die.The effect of this process is to extrude cylinder 61 along its length.By a further modification of this apparatus, the series of cylinderseach rotate about an axis perpendicular to the axis of cylinder 61.

Another form of coil fabrication process will now be described withreference to FIGS. 11 and 13-15. In this embodiment, a textured cylinder71 (the cylindrical surface only of which is shown in FIG. 13) isco-wound with two tungsten wire heater elements, one of which isimmediately unwound leaving an evenly-spaced heater winding 72 in placein contact with the surface of the cylinder, and a “spiral gap” 73elsewhere. The heater winding 72 acts as a “shadow mask” during theprocess described below.

The cylinder 71 is then placed inside the deposition chamber 51 of theapparatus of FIG. 11, and is coated with a first buffer layer 74 (seeFIG. 13) using the heater winding 72 to achieve the correct depositiontemperature. As with the earlier film deposition embodiments, thecylinder 71 is rotated during deposition. The buffer layer 74 isdeposited using a thermal evaporation source, as indicated by the arrowsshown in FIG. 13. This process copies the texture from the cylinder 71.Obviously, that part of the cylinder 71 masked by the heater winding 72does not receive a buffer layer, so that the situation at this stage isas illustrated in the cross-sectional view of FIG. 14. The buffer layer74 thus forms a spiral track between the turns of the heater winding 72.

The cylinder 71 is then moved into the chamber 52 of the apparatus ofFIG. 11, where it receives a YBCO coating, again with that part of thecylinder underneath the heater winding 72 being left uncoated. Thisprocess copies the texture from the buffer layer 74 to the YBCO layer(not shown in FIGS. 13 and 14). The cylinder 71 is then passed back tothe chamber 51, where it receives another buffer layer on top of thespiral YBCO track which then exists. The cylinder 71 is then passed tothe chamber 53, where it is re-wound with the second heater element (notshown), after which the first heater winding 72 is removed, therebyleaving the spiral YBCO track with an insulating textured buffer layeron top, as the first “coil winding”. Spaces are left in between thespiral coil winding turns which are uncoated, bare cylinder of the samedimensions as the spiral track.

The cylinder 71 is then returned to the chamber 51 and receives anotherinsulating buffer layer. This layer is deposited on top of thepreviously-masked part of the bare cylinder. Subsequently, a YBCO filmis deposited in the chamber 52, and the cylinder 71 is then passed backto the chamber 51 for application of another buffer layer. The cylinder71 is then passed to the chamber 53, where the first heater winding isre-wound and the second heater winding is removed. Connections betweenthe first and second windings can be made using a similar scheme to thatshown in FIG. 8, this process being carried out in the chamber 53. Atthis point, there are two YBCO layers in the form of windings, both withtextured insulating buffer layers on top. One of them is covered with aheater element, the other is not. The whole process can now begin again,changing the heater element position each time so that a multi-layerstructure of YBCO “windings” is formed, with the necessary connectionsbetween them. The windings are, in effect, interlaced. This position isshown schematically in FIG. 15, which show the cylinder 71, the bufferlayers 74 and the YBCO layers 75 of a finished coil formed in thismanner.

The diameter of the heater windings maybe typically 200-500 microns,whereas the layer thicknesses are generally of the order of a fewmicrons. In this connection, it will be appreciated that FIGS. 13-15 arenot drawn to scale; but, if the sources for thermal evaporation are somecentimetres away, this gives a sharp masking effect with very little“penumbra” effect. It will also be appreciated that, during depositionssteps subsequent to the first, the position of the next heater windingwill be displaced exactly by the diameter of the wire of the heaterwindings.

The structure shown in FIG. 15 forms a simple, four-layer coil, and thelayers must be connected, either in series or in parallel, dependingupon the preferred power supply. Thus, for a series connection, thecylinder ends can be in the form of film FCLs (either pre-deposited orpost-deposited), such that the layers are connected through the FCL filmdevices. In order to maintain the same direction of current through thespiral YBCO tracks comprising the “coil windings”, each layer must beconnected to the opposite end of the next layer, via some internalsuperconducting link via a respective FCL device.

As an alternative to the scheme described above, after the first YBCOlayer and buffer overcoat have been deposited, the sense of the nextheater winding is reversed so that successive YBCO layers can beconnected to each other without the type of axial superconductinglengths shown in FIG. 8. This gives the effect of a “layer turn” in aconventional wire-wound solenoid. The essential point here is that theoriginal texture of the cylinders surface must be copied with eachsuccessive layer, whatever the precise geometry or manufacturingprocess.

The heater windings themselves may be deposited using film depositiontechniques at each stage, and can be removed using an etching processafter photolithography. In this sense, they would be “sacrificial”rather than reusable as was the case with the process described above.

Another variation, would be to cut a spiral groove in the texturedcylinder surface and to use this to embed a heater winding, with thewinding standing proud of the surface. A textured multilayer structureis then built up on the adjacent sections, geometrically similar to thescheme of FIG. 8, and the heater winding can be removed after the finaldeposition stage. This simplifies the process, but does not achievequite the same current density as the co-wound, double heater windingapproach described above.

Finally, the delineation of the coils may be accomplished usingphotolithographic techniques, and a careful choice of the chemicaletchants to distinguish between etching of the buffer and YBCO layers.In this way, “etch stop” techniques can be used, which increases thetolerance of the processing.

In any of the techniques described above, film deposition can beaccomplished using the so-called “dip coating” or other “thick film”technique, whereby the film is supplied by rotating the cylinder in asolution of, for example, sol-gel of cerium oxide.

The coil fabrication techniques above all refer to the use of asubstantially cylindrical former. As a modification of this fabricationtechnique, coils can be manufactured upon any curved surface, includinga saddle, a cone, a surface having a concave rather than a convexsurface, a surface with negative curvature rather than a surface withpositive curvature, and a surface that does not have an axis ofrotation.

The films that are described in the above methods are of the order ofmicrometers. Typically thin films, as used in the semiconductor industryor for optical coatings, are a few hundred nanometers thick. Thickfilms, on the other hand, as used in printed circuit boards, are tens tohundreds of microns thick. Here, the HTS films fall between thedefinitions of thick and thin films.

Since the HTS and buffer films have a thickness that lies between thedefinition of thin films and thick films, both thin film depositionmethods such as evaporation, sputtering, MOCVD, and thick filmdeposition methods, such as “dip-coating”, spray pyrolysis and, spincoating can be used to make HTS films. However, in practice, it shouldbe borne in mind that a concentric solenoid with 100 thick layers islikely to be easier to manufacture than a concentric solenoid with 1000thin layers.

There a number of very important consequences of film depositiontechniques which do away with the superconducting wire and windingprocesses which are traditionally used in manufacturing coils, namely:—

-   1. The definition of a coil, using planar technology such as    photolithography, etching etc, can be done after the film    deposition, rather than as part of the winding process. This lessens    the mechanical constraints such as the introduction of defects by    bending the wire etc, which are inevitable through handling and coil    winding procedures. It also means that the same basic coated formers    may be used to fabricate a variety of structures—e.g. coils of    different track width, different “winding pitch”, or even variable    winding pitch to modify and control fields more precisely. The    latter is an important point, as normally the winding of a coil is    constrained by the diameter of wire, length of coil, number of turns    etc, but the new processes described above give extra degrees of    freedom for end corrections, to take but one example. The geometry    of the “windings” in the new approach is obviously much more    flexible, and not constrained by the diameter of the wire etc.-   2. Coil windings are conventionally impregnated with resin to stop    them from moving under the high magnetic fields generated. A high    field superconducting magnet can be regarded as a pressure vessel    subject to enormous forces trying to explode the containment. Hence,    there are many reinforcing strategies. The overall mechanical    integrity of the windings is an important factor in nearly all    applications, and again the new ideas, which essentially embody an    “integrated manufacturing approach” are a significant advance. Thus,    each layer of coil is bonded to each underlying layer by the    immensely strong atomic bonding forces rather than by friction,    tension etc as in conventional windings. Control of tension and    angles of wire during winding are critical factors which are    completely eliminated using the new approach. Also, a former with    initial grooved structure might also give added strength to the    overall component. Such grooves may also serve to “register”    subsequent layers by use of a “throwaway” heater winding approach    described above.-   3. Conventionally, insulation is introduced alongside the wire    either in a “co-wound” configuration or as a sleeve. In the new    approach, the insulation using films as “buffer layers” is an    integral part of the structure. Typically, each layer of YBCO will    be sandwiched between textured insulating layers. The texture is    copied from one layer to another, and film technology minimises the    number of defects in both insulating and superconducting layers.    This is comparable to the case of silicon technology, where many    different layers comprise the overall circuit. However, in the    present case, the texture of the layer must be copied from one layer    to another as in the case of a solid state laser heterostructure.-   4. A “layer verification” strategy can be incorporated, whereby XRD    (X-ray diffraction) and ion beam or electron beam techniques (such    as ion beam or electron channelling patterns) are used to verify    each layer as it is being formed. Also, since the film deposition is    likely to be performed in a vacuum chamber, it would be possible to    verify the superconducting properties of the coil during manufacture    by cooling the coil, e.g by passing liquid nitrogen through a hollow    shaft or channels in the former supporting the coil, and by    measuring the electrical/magnetic properties to verify each    superconducting layer, for example. Since the final fabricated coil    must also be cooled in use, this incorporation of channels for    liquid or gaseous coolant is another example of the “integrated    manufacture” approach, with the vacuum chamber serving as a    temporary cryostat for in-situ measurements at low temperatures.-   5. The mechanical integrity of the resulting coils will far exceed    that of existing coils, because no windings can move, delaminate    etc.-   6. Winding with coated conductor results in “wasted” material not    carrying substantial current—i.e. the substrate thickness is wasted    in conventionally-wound coils. With the thin/thick film deposition    process described above, the “substrate” is a cylinder which has    considerably greater mechanical strength (compared with conventional    nickel or nickel alloy substrates), so the deposition process is    superior from both mechanical and electrical reasons, to the use of    wound coated tape.

Although specific materials have been referred for use as the HTS films,any material—particularly magnesium diboride or ReBCO (of which YBCO isa common example, as Re denotes any rare earth element)— which, as afilm, exhibits high temperature superconducting properties could be usedfor the manufacturing processes and articles described herein.

Similarly, any material which exhibits the physical properties of thebuffer layers can be used in buffer layers acting, for example, as seedlayers, insulators, or chemical barriers or combinations of these in themanufacturing processes and articles described herein. It is also commonto have more than one buffer layer underneath the HTS film layer.

Specific process and approaches have been referred to above: such asRABiTS, in the context of ensuring a textured HTS layer whereby thenecessary biaxial texture is imparted to a metallic substrate by acombination of rolling reductions and heat treatments; and in othercases IBAD, ISD and IAD, where the necessary biaxial seed layer isproduced on a randomly-oriented substrate by depositing a buffer layerin such a way that it is textured. The references to those processes andapparatus also include variants of those specific processes. Suchvariants of those processes can include: texturing a cylinder byextrusion or melt texturing instead of rolling, or by molecular beamepitaxy (MBE), or hot dipping in a suitable flux. The texturing processmay be driven by gradients in pressure, temperature or other physicalparameters.

As described above, the present invention provides a method offabricating a superconducting coil, the method comprising the step offabricating individual coil windings by depositing and shapingelectrically-conductive material in situ on a substantially cylindricalformer.

Advantageously, the method further comprises the step of depositing andshaping buffer layers between successive coil windings.

Preferably, the electrically-conductive material is deposited on theformer by a thin film deposition technique, and the buffer layers aredeposited by a thin film deposition technique.

In a preferred embodiment, the method comprises an intial step offorming a spiral of textured buffer layer on the former and the texturedbuffer layer is formed by helically positioning a flexible textured tapeonto the former.

In another preferred embodiment, the spiral textured buffer layer isformed by this thin deposition technique.

In a preferred embodiment, the thin film deposition technique includesthe step of forming a spiral of textured buffer layer on the former,depositing an electrically-conductive layer over the spiral buffer layerto form a first coil winding, depositing a second buffer layer onto theelectrically-conductive layer, and depositing a secondelectrically-conductive layer onto the second buffer layer, therebyforming a second winding of the coil, and repeating the buffer layer andelectrically-conductive layer depositions to form as many coil windingsas required, each deposition process being such as to transfer thetexture of the underlying layer to the newly-deposited layer. In thiscase, the spiral of textured buffer layers may be written onto theformer using the IBAD technique using a fixed ion beam and rotating andtranslating the former. Alternatively, the ion beam is translated andthe former is rotated but not translated. Another possibility would beto form a buffer layer completely overlying a textured cylindricalformer, and then removing a spiral track of the buffer layer (forexample lithographically) to form the spiral of textured buffer layers.

Conveniently, each of the electrically-conductive layers is an YBCOlayer. Alternatively, each of the electrically-conductive layers is anyother suitable superconducting film such as a rare earth barium copperoxide (ReBCO) film.

Preferably, the method further comprises providing connecting links toconnect the ends of adjacent coil windings. Conveniently, the connectinglinks are provided within the former, and each includes a fault currentlimiter.

Advantageously, the deposition steps take place in a film depositionchamber, and deposition of the buffer layers occurs from one side of thechamber, and deposition of the electrically-conductive layers occursfrom the opposite side of the chamber, and the former is rotated duringthe deposition process. The two sides of the deposition chamber may beseparated by baffles.

In another preferred embodiment, the former is provided with a texturedcylindrical surface, and with a spirally-wound heater winding wire, theturns of which are spaced by the diameter of the wire, a first bufferlayer is deposited to form a spiral buffer layer track between the turnsof the heater winding, an electrically-conductive layer is depositedover the first buffer layer, and a further buffer layer is deposited ontop of the electrically-conductive layer to form a first coil winding, asecond heater winding wire is wound between the turns of the firstheater winding wire, the first heater winding is removed, and a secondcoil winding is formed, the second coil winding being constituted by afirst deposited buffer layer, a deposited electrically-conductive layerand a second deposited buffer layer, and the process is repeated to formadditional coil windings as required, each deposition process being suchas to transfer the texture of the underlying layer to thenewly-deposited layer.

The method may further comprise the step of circulating coolant withinthe former, and/or of testing, in situ, each coil winding.

The invention also provides a method of making a multilayered texturedsuperconducting coil, the method including the steps of fabricatingsuperconductor coil windings and insulating layers by film depositionmeans, whereby the films are patterned by masking or machiningoperations before or after film deposition, in-situ or ex-situ, and/orby subsequently patterning using lithographic techniques allowing atailoring of coil properties by changing the geometry (width, thickness)of the superconducting paths at every point on the surface.

The invention further provides a method of fabricating a superconductingcoil, the method comprising the step of helically positioning a flexiblecoated tape onto a substantially cylindrical former, the coated tapebeing constituted by an insulating buffer layer and a coating ofelectrically-conductive material.

Advantageously, YBCO constitutes the electrically-conductive material,and the tape is a textured substrate made, for example, by the RABiTSprocess.

Preferably, the former is generally barrel-shaped, with tapered portionsat each end of a cylindrical control main portion.

1. A method of fabricating a superconducting coil, by fabricatingindividual coil windings by depositing, shaping and texturingsuperconductive material and buffer material on a former which has asubstantially curved surface, the method comprising the steps of:forming a textured buffer layer on the former; depositing a first layerof superconductive material over the textured buffer layer so as to copythe crystallographic texture of the textured buffer layer to the firstlayer of superconductive material; depositing a second buffer layer overthe first layer of superconductive material so as to copy thecrystallographic texture of the first layer of superconductive materialto the second buffer layer; depositing a second layer of superconductivematerial over the second buffer layer so as to copy the crystallographictexture of the second buffer layer to the second layer ofsuperconductive material; depositing further buffer layers and layers ofsuperconductive material alternately so as to copy the crystallographictexture of each underlying layer to each overlying layer, thedepositing, shaping and texturing forming successive coil windings inthe layers of the superconductive material; forming a reinforcementshell on the coil after depositing the layers of superconductivematerial; forming a textured layer on the surface of the reinforcementshell to define a further buffer layer using the same process as used onthe textured buffer layer; forming further coil windings by the additionof sequential superconducting and buffer layers; and providingconnecting links to connect the ends of adjacent coil windings.
 2. Amethod as claimed in claim 1, wherein the former defines a substantiallycylindrical surface.
 3. A method as claimed in claim 2, wherein theformer defines a substantially right circular cylindrical surface.
 4. Amethod as claimed in claim 1, wherein each layer of superconductivematerial is deposited by a film deposition technique.
 5. A method asclaimed in claim 1, wherein each of the textured buffer layers isdeposited by a film deposition technique.
 6. A method as claimed inclaim 5, further comprising an initial step of forming a spiral oftextured buffer layers on the former.
 7. A method as claimed in claim 5,further including the steps of fabricating superconductor coil windingsand insulating layers by film deposition means, whereby each film ispatterned by masking or machining operations before or after filmdeposition by the film deposition means.
 8. A method as claimed in claim5, further including the steps of fabricating superconductor coilwindings and insulating layers by film deposition means, whereby thefilms are patterned in-situ.
 9. A method as claimed in claim 5, furtherincluding the steps of fabricating superconductor coil windings andinsulating layers by film deposition means, whereby the films arepatterned using lithographic techniques allowing a tailoring of coilproperties by controlling the geometry of the superconducting paths atevery point on the surface.
 10. A method as claimed in claim 1, whereinthe textured buffer layer is formed by helically positioning a flexibletextured tape onto the former.
 11. A method as claimed in claim 1,wherein the textured buffer layer is formed by a film depositiontechnique.
 12. A method as claimed in claim 1, wherein the texturedbuffer layer is written onto the former using the IBAD technique using afixed ion beam and rotating and translating the former.
 13. A method asclaimed in claim 12, further including the steps of fabricatingsuperconductor coil windings and insulating layers by film depositionmeans, whereby each film is patterned by masking or machining operationsbefore or after film deposition by the film deposition means.
 14. Amethod as claimed in claim 12, further including the steps offabricating superconductor coil windings and insulating layers by filmdeposition means, whereby the films are patterned in-situ.
 15. A methodas claimed in claim 12, further including the steps of fabricatingsuperconductor coil windings and insulating layers by film depositionmeans, whereby the films are subsequently patterned using lithographictechniques allowing a tailoring of coil properties by controlling thegeometry of the superconducting paths at every point on the surface. 16.A method as claimed in claim 1, wherein each of the layers ofsuperconductive material is a YBCO layer.
 17. A method as claimed inclaim 1, wherein the connecting links are provided within the former.18. A method as claimed in claim 1, wherein each of the connecting linksincludes a fault current limiter.
 19. A method as claimed in claim 1,wherein all the depositing steps take place in a film depositionchamber.
 20. A method as claimed in claim 19, wherein the former isprovided with a spirally-wound heater winding wire, the turns of whichare spaced by a means for spacing, said textured layer is deposited toform a spiral track between the turns of the heater winding, the firstlayer of superconductive material is deposited over the textured layer,and the first buffer layer is deposited on top of the first layer ofsuperconductive material to form a first coil winding, a second heaterwinding wire is wound between the turns of the first heater windingwire, the first heater winding is removed, the second layer ofsuperconductive material is deposited on top of the first buffer layerto form a second coil winding, and the process of providing windingheater wires and depositing buffer layers and layers of superconductivematerial is repeated to form additional coil windings as required, eachdeposition process transferring the crystallographic texture of theunderlying layer to a newly-deposited layer.
 21. A method as claimed inclaim 20, wherein the means for spacing the turns of the spirally-woundheater winding wire is the diameter of the wire.
 22. A method as claimedin claim 20, wherein the means for spacing the turns of thespirally-wound heater winding wire is by grooves in the former.
 23. Amethod as claimed in claim 20, further comprising the step ofcirculating coolant within the former.
 24. A method as claimed in claim20, further comprising the step of testing, in situ, each coil windingin terms of texture or superconducting performance.
 25. A method asclaimed in claim 1, wherein the fabrication of a multilayered texturedsuperconducting coil includes the steps of fabricating superconductorcoil windings and insulating layers by film deposition means, wherebyeach film is patterned by masking or machining operations before orafter film deposition by the film deposition means.
 26. A method asclaimed in claim 1, wherein the fabrication of a multilayered texturedsuperconducting coil includes the steps of fabricating superconductorcoil windings and insulating layers by film deposition means, wherebythe films are patterned in-situ.
 27. A method as claimed in claim 1,wherein the fabrication of a multilayered textured superconducting coilincludes the steps of fabricating superconductor coil windings andinsulating layers by film deposition means, whereby the films arepatterned using lithographic techniques allowing a tailoring of coilproperties by controlling the geometry of the superconducting paths atevery point on the surface.
 28. A method of fabricating asuperconducting coil, by fabricating individual coil windings bydepositing, shaping and texturing superconductive material and buffermaterial on a former which has a substantially curved surface, themethod comprising the steps of: forming a textured buffer layer on theformer; depositing a first layer of superconductive material over thetextured buffer layer so as to copy the crystallographic texture of thetextured buffer layer to the first layer of superconductive material;depositing a second buffer layer over the first layer of superconductivematerial so as to copy the crystallographic texture of the first layerof superconductive material to the second buffer layer; depositing asecond layer of superconductive material over the second buffer layer soas to copy the crystallographic texture of the second buffer layer tothe second layer of superconductive material; depositing further bufferlayers and layers of superconductive material alternately so as to copythe crystallographic texture of each underlying layer to each overlyinglayer, the depositing, shaping and texturing forming successive coilwindings in the layers of the superconductive material; forming areinforcement shell on the coil after depositing the layers ofsuperconductive material; forming a textured layer on the surface of thereinforcement shell to define a further buffer layer using the sameprocess as used on the textured buffer layer; and forming further coilwindings by the addition of sequential superconducting and bufferlayers, wherein all the depositing steps take place in a film depositionchamber, and wherein the depositing of the buffer layers occurs from oneside of the deposition chamber, and the depositing of the layers ofsuperconductive material occurs from the opposite side of the depositionchamber, and the former is rotated during each deposition step.
 29. Amethod as claimed in claim 28, wherein the two sides of the depositionchamber are separated by baffles.
 30. A method of fabricating asuperconducting coil, by fabricating individual coil windings bydepositing, shaping and texturing superconductive material and buffermaterial on a former which has a substantially curved surface, themethod comprising the steps of: forming a textured buffer layer on theformer; depositing a first layer of superconductive material over thetextured buffer layer so as to copy the crystallographic texture of thetextured buffer layer to the first layer of superconductive material;depositing a second buffer layer over the first layer of superconductivematerial so as to copy the crystallographic texture of the first layerof superconductive material to the second buffer layer; depositing asecond layer of superconductive material over the second buffer layer soas to copy the crystallographic texture of the second buffer layer tothe second layer of superconductive material; depositing further bufferlayers and layers of superconductive material alternately so as to copythe crystallographic texture of each underlying layer to each overlyinglayer, the depositing, shaping and texturing forming successive coilwindings in the layers of the superconductive material; forming areinforcement shell on the coil after depositing the layers ofsuperconductive material; forming a textured layer on the surface of thereinforcement shell to define a further buffer layer using the sameprocess as used on the textured buffer layer; forming further coilwindings by the addition of sequential superconducting and bufferlayers; and providing connecting links between the layers of thesuperconductive material.